The present invention relates to an improved device and method for monitoring the thread reserve in devices for feeding weft to looms and textile machines in general.
More particularly, the invention relates to conventional weft feeders comprising a fixed drum on which a windmilling rotating arm winds a plurality of turns of thread constituting a reserve of weft, in which the turns of the reserve are unwound in a preset amount at each beat of the loom, and in which sensor means are provided which are capable of starting and stopping the motor that actuates the windmilling arm when the thread reserve drops below a preset number of turns and, respectively, when the reserve has been fully restored or if the thread breaks.
European patent no. 0 171 516 discloses a weft feeder of the specified type, in which the amount of thread reserve, which can vary between a minimum value and a maximum value, is monitored by means of at least one thread reserve sensor mounted in the fixed accumulation drum so as to be movable, in contrast with a return force, between a first position, which protrudes beyond the surface of the drum and in which the sensor is arranged when there are no turns of thread, and a second position, in which said sensor, actuated by the thread, is arranged at the same level as the surface of the fixed accumulation drum, and in which the sensor cooperates with a switching device located outside the accumulation drum and reacting with a signal, without contact, when the position of the sensor changes. Typically, the thread presence sensor is constituted by a permanent magnet and the switching device is sensitive to the variation in the magnetic field that occurs when the sensor passes from the first position to the second position and vice versa.
A drawback of this conventional system for monitoring the amount of weft reserve is the fact that the signal of the switching device can vary even significantly from one device to another, due both to the different relative position of the sensor and of the cooperating switching device and to the unavoidable variations in the parameters of the components, and this makes it difficult to calibrate the system and can cause false activations.
Another drawback is the fact that when the reserve of weft ends before the sensor, the turns that unwind from the drum strike said drum, causing it to move downwardly because of the limited contrast force applied thereto. These downward motions of the sensor cause a corresponding variation in the output signal of the switching device, which can be interpreted as a signal indicating that a reserve is present when this condition actually is not occurring. These false signals can easily lead the control system to unstable conditions, with the consequence that the feeding of the thread on the fixed drum of the feeder does not occur uniformly but is characterized by sudden accelerations and brakings that can easily break the thread.